The present invention relates to a semiconductor device, particularly to a semiconductor device equipped with MOS transistors.
Silicon oxide is a material having excellent insulating properties, featuring a bandgap of as large as 8.0 eV. Thanks to its high insulating performance, this material has been dominantly used for insulating films such as gate insulators and layer insulators in semiconductor devices.
In recent years, however, with miniaturization of semiconductor devices, thinning of the gate insulators has become an essential requirement, and oxide films with a thickness of less than 3.0 nm have come to be used as such insulators. Decrease of insulating film thickness to less than 3.0 nm entails an inegligibly high rise of tunneling current, causing a corresponding increase of leakage current and power consumption.
It is known that tunneling current can be classified roughly into two types: Fowler-Nordheim tunneling current (FN current) and direct tunneling current (DT current). FN current is a current which is caused to flow as the electrons tunnel the triangular potential produced as a result of distortion of the energy barrier by an external electric field. DT current is a current which is caused to flow by tunneling of the electrons directly through the oxide films.
It has been disclosed that ideally FN current and DT current can be defined by using the WKB (Wentzel-Kramers-Brillouin) approximation according to, for example, the equation (A1) on page 354 of IEEE TRANSACTIONS ON ELECTRON DEVICE, Vol. 46, No. 2, making use of energy barrier ΦB between electrode and insulating film.
Rise of tunneling current by the thinning of silicon oxide gate insulators is primarily attributable to the increase of DT current. There is a concern that when various types of high-permittivity material are used for the gate insulators, the leakage current density may elevate sharply depending on the producing method of gate electrodes or gate insulators.